1. Field of the Invention
The present invention relates to an electron gun for a color cathode-ray tube for use in a large television set or a high-definition monitor, and more particularly to an improvement in the deterioration of resolution on the peripheral regions of the screen.
2. Description of the Prior Art
A conventional color display system, as shown in FIG. 1, is composed of a cathode-ray tube and a deflection yoke. The tube is composed of a panel and a funnel in which an electron gun is installed in a neck of the funnel.
Generally, the electron gun consists of a beam-forming region and a main lens. The beam-forming region is composed of cathodes 2a,2b and 2c, a G1 (control) electrode and a G2 (accelerator) electrode 4. Thermions activated by the cathodes are controlled and accelerated by the G1 and the G2 electrodes, respectively, to form electron beams. A voltage of about 0 V is applied to the G1 electrode, and a voltage of about 500 V to 700 V is applied to the G2 electrode.
The main lens is composed of a G3 (focus) electrode 5 and a G4 (anode) electrode 6. The high voltage of about 25 KV to 30 KV is applied to the G4 (anode) electrode, and an intermediate voltage of about 20% to 30% of the G4 (anode) voltage is applied to the G3 electrode. According to the difference between the G3 electrode voltage and the G4 electrode voltage, an electrostatic lens is formed. This electrostatic lens functions to focus the electron beams formed by the beam-forming region on the screen of the tube.
Generally, the focused electron beams consists of three electron beams of red(R), green(G), and blue(B) colors. In a color cathode-ray tube using an in-line type electron gun, a self-convergence magnetic field, which is a nonuniform deflection magnetic field of the deflection yoke, is produced in order to focus the three electron beams of R, G, and B colors onto one spot. FIGS. 3A and 3B show the distribution of the self-convergence magnetic field formed as above. This magnetic field may be separated into a dipole component and a quadrupole component as shown in FIGS. 3C and 3D. The dipole component effects a main deflection of the electron beams in a horizontal direction, while the quadrupole component forces the electron beams to be converged in a vertical direction and to be diverged in a horizontal direction, causing astigmatism to be developed.
As shown in FIG. 5, spots of the electron beams are different in the horizontal and vertical directions and overfocused on the screen, resulting in that deterioration in resolution increases with the distance from the center of the screen to the peripheral portion of the screen. Such a nonuniform magnetic field consists of a pin cushion magnetic field and a barrel magnetic field as shown in FIGS. 3A and 3B. Accordingly, as shown in FIG. 4, the shape of the electron beam in the peripheral portion of the screen goes with haze in a vertical direction, thereby deteriorating resolution in the peripheral portion of the screen.
In order to solve the above-mentioned problems, there have been various types of electron guns utilizing a dynamic quadrupole electrode for compensating for the astigmatism when the electron beam is deflected to the peripheral portion of the screen, as shown in Blacker et al. U.S. Pat. No. 4,771,216, Osakaba U.S. Pat. No. 4,772,827, Bloom et al. U.S. Pat. No. 4,887,009, Chen et al. U.S. Pat. Nos. 5,036,258 and 5,055,749, Suzuki et al. U.S. Pat. No. 5,061,881, and Bae et al. U.S. Pat. No. 5,281,896. According to the above patents, a quadrupole lens or a multipole lens is formed by installing a quadrupole electrode or a multipole electrode between the beam-forming region and the main lens to compensate for astigmatism. In forming the quadrupole lens, a G3 (focus) electrode is separated into a first focus electrode and a second focus electrode, and the quadrupole electrode for forming the quadrupole lens is provided between the first and second focus electrodes. A static focus voltage is applied to the first focus electrode and a dynamic focus voltage, which varies according to the deflection amount of the electron beam, is applied to the second focus electrode. Generally, the dynamic focus voltage is determined to be higher than the static focus voltage. As shown in FIG. 20, the dynamic focus voltage is applied with a horizontal parabolic waveform and a vertical parabolic waveform in accordance with the scanning direction of the electron beam.
U.S. Pat. No. 4,771,216 discloses an electron gun in which rectangular beam-passing apertures are formed on a first focus electrode (i.e., a static electrode), and partitions are formed on a second focus electrode so as to be located on the upper and lower portions of the beam-passing apertures as shown in FIG. 8. The rectangular beam-passing apertures and the partitions located on the upper and lower portions of the beam-passing apertures constitute a quadrupole lens to compensate for astigmatism.
U.S. Pat. No. 4,772,827 discloses an electron gun in which a quadrupole lens is formed between first and second focus electrodes as shown in FIG. 10. The first focus electrode has vertical partitions formed thereon in a horizontal direction of the electron beams, and the second focus electrode has horizontal partitions formed thereon in a vertical direction of the electron beams. The horizontal and vertical partitions constitute a quadrupole lens for compensating for astigmatism.
U.S. Pat. No. 4,887,009 discloses an electron gun in which first and second focusing electrodes have burrings extended therefrom, respectively, as shown in FIG. 7. The burrings of the first and second focusing electrodes are engaged with each other and constitute a quadrupole lens for compensating for astigmatism.
U.S. Pat. Nos. 5,036,258 and 5,055,749 discloses an electron gun in which a key-hole-shaped quadrupole electrode is formed between first and second focus electrodes to compensate for astigmatism as shown in FIG. 9.
U.S. Pat. No. 5,061,881 discloses an electron gun in which beam-passing holes formed on a first focus electrode are in the shape of a vertically-elongated rectangle and those formed on a second focus electrode are in the shape of a horizontally-elongated rectangle to form a quadrupole lens for compensating for astigmatism.